Methods of this type are already known in which a concentrated light beam is directed onto the surface under investigation and the beam reflected, or more precisely backscattered, by the surface is observed. Depending on the relief or the roughness of the surface, the reflected beam diverges to a greater or lesser extent, and a representation of the relief or surface state can be obtained by studying said divergence.
One such prior method is described in patent document SU-A-1 067 350 in which the surface under investigation is iluminated at a certain angle and the radiation of the reflected beam is measured at various points in a plane perpendicular to the direction of specular reflection.
This measurement serves to determine the energy distribution of the reflected ray, and thus to obtain values or curves representative of the reflected radiation which, by comparison with values established at other points on the surface, or on a reference surface, can be used to determine the roughness or relief characteristics at the location of the surface under investigation.
It should be observed that a measurement performed using the above method is characteristic of one point on the surface under investigation, and that an examination of the entire surface requires repetitive measurements to be performed at all points on said surface, and that each measurement may require several operations to be performed. Alternatively, a certain area of the surface must be illuminated, in which case the resulting measurement is "averaged" over said area.
Further, measuring the relief of a circuit using this method is essentially comparative, and therefore the shape or profile of said relief is not accurately determined. Further, the result of the measurement is subject to variations in the radiation emitted by the source, which variations may occur while a measurement is taking place or between two consecutive measurements.
U.S. Pat. No. 4,017,188 describes a method in which the surface under investigation is illuminated with a light beam, in which the beam backscattered by said surface is focussed by means of a lens, in which the beam from said lens is split into a measuring beam which is passed through a diaphrahm and which possesses a total energy of value Wf after passing through the diaphragm, and a reference beam which is not passed through a diaphragm and which possesses a total energy of value Wo. The incident beam and the lens are displaced relative to the surface under investigation at a constant average distance from said surface, and throughout this displacement the energies of the measurement beam and the reference beam are continuously measured, and for each measurement point the ratio Wf/Wo of their respective values is determined, with variation in said ratio during the above-described displacement being representative of the relief of the surface under investigation. According to said US document, the diaphragm is constituted by a slot, thereby limiting the application of the method to particular types of relief. Also, and above all, the system described places the lens between the light source and the surface under investigation. The resulting double defocussing effect leads to a small light spot and consequently to the measured area being considered reduced: any defocussing of the impact of the light beam on the surface under investigation gives rise to a corresponding limit in the spatial resolution in a direction parallel to said surface.
The object of the present invention is to provide a method of determining or measuring the relief of a surface, which method is capable of mitigating the above-mentioned drawbacks.